This invention relates to the manufacture of extruded elongated products, especially but not exclusively, electric cables and pipes, that are of carbon chain polymers cross-linked by the use of hydrolysable unsaturated silane. In the past such products have been formed by first reacting the polymer with the hydrolysable unsaturated silane in the presence of a free-radical generator such as a peroxide and afterwards exposing the grafted material to the effects of moisture and heat and a silanol condensation catalyst. Suitable reagents for this process have been described in U.S. Pat. No. 3,646,155 which issued on Feb. 29, 1972. Since the issuance of that patent it has become known that this process is applicable to a variety of polymers other than polyethylene and the modified polyethylenes referred to in that specification. Examples of such other polymers are chlorinated polyethylene and a wide range of olefin copolymers which can be processed and in the practice of the present invention the reaction conditions set forth in the above identified U.S. patent need not in all cases be strictly adhered to. In the process of the above identified patent the polyethylene is mixed with an organic silicon derivative in the presence of an anti-oxidant and a peroxide chemical interaction occurs upon that mixture. The purpose of this operation is to "graft" a free silane radical onto the linear polyethylene chain. This grafting process takes place whenever the polyethylene and the organic silicon derivative comes into contact but the product must be dried before it is mixed with an appropriate catalyst which must be stored separately. Thereafter the cross-linked graft co-polymer is mixed with the catalyst and the resulting product is extruded onto a cable or into a suitable elongated product.
The extruded product is subsequently cross-linked by immersion in hot water or saturated steam and the operation can take several hours depending upon the thickness of the insulation and the temperature of the water or steam.
An improvement on the above identified process is disclosed in U.S. Pat. No. 4,117,195 issued on Sept. 26, 1978 to Peter Swarbrick et al. In this process the grafting and the mixing states are combined into a single operation and the preliminary agranulation step is eliminated. In this process polyethylene or another suitable polymer which is capable of being cross-linked by the use of a hydrolysable unsaturated silane is metered into a screw extrusion machine together with the compounding ingredients which comprise a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst. All of these ingredients are blended with the polymer in the barrel of the extruder and then the temperature of the mixture is raised sufficiently to effect grafting of the silane groups to the polymer. The mixture is then melted and extruded from the extruder through an extrusion die to form an elongated shaped product. This product is then cross-linked by the action of moisture.
While the method of U.S. Pat. No. 4,117,195 is an improvement over the method of U.S. Pat. No. 3,646,155, it still has a number of disadvantages. For example, this process requires a very expensive screw extrusion machine which has a very long barrel to give the material time to mix, melt, homogenize, and graft within the barrel of the extruder. For example, this process requires an extruder which has a length to diameter ratio of 28 or 30 to 1 in order to produce a melted, blended polymer ready for extrusion and for cross-linking. It is believed that one reason for this is that the heating is very erratic where the liquid is injected with the solid particles into the barrel or the hopper before the polymer is heated. Thus it is believed that uneven grafting takes place before the polymer begins to melt or is melted to any substantial degree.
In addition to the drawbacks noted before, the process of U.S. Pat. No. 4,117,195 also requires very expensive dosimetry equipment for each ingredient fed into the hopper of the extruder since the polymer and each of the compounding ingredients must be carefully metered into the extruder so as to provide the proper ratio of compounding ingredients and the polymer.
Additional drawbacks to the prior art process is the requirement that the manufacturer utilizing this process is required to purchase, store and handle a multiplicity of ingredients.